The structural basis of MHC antigen presentation represents one of the major focuses of immunology. We are currently pursuing studies on molecules of the T cell that are involved in T cell recognition directed at understanding the rules by which the ?? T cell receptor molecules interact with the peptide bound in the MHC molecule and the subsequent events that direct T-cell activation. In addition to the MHC and T-cell receptor, a number of proteins such as CTLA-4 and B7 are essential for modulating the T-cell response. We have prepared crystals of the co-stimulatory factor CTLA-4 and have collected excellent data at X9B to 2.0E resolution. We are currently using the outstanding tunability at the X9B station to solve this structure by MAD using conventional heavy atom derivatives and selenomethionine substitution.